The use of carbide tipped cutting tools has greatly advanced machining practices in recent years due to the advantages brought about with the introduction of replaceable carbide inserts for the cutting tools, to provide a plurality of cutting edges on each insert which are indexibly supported on a tool holder in a suitable cutting orientation. These replaceable inserts eliminate the need to resharpen the cutting edges and typically include 90.degree. lip angles, providing mechanically strong cutting edges for the brittle carbide inserts. The turning applications of these indexible carbide inserts, however, have been somewhat limited until the present invention. Due to the turn-over and rotational procedure for indexing the inserts to accommodate the placing of each of the cutting edges in an operable cutting position, all of the cutting edges must be identical, necessitating 90.degree. lip angles.
Since it has long been settled and fully accepted by those skilled in the art, as exemplified by engineering handbooks and trade publications, that the "standard" relief angle of 6.degree. is virtually a mandatory minimum clearance angle and by fixing the relief angle as not less than 6.degree., a cutting angle of 96.degree. or more is automatically set which places the rake angle at a negative rake of 6.degree. or more. It has been shown that such a severe negative rake angle could be tolerated only in some turning operations, but could not produce the high quality finish required for wheel bearing journals.
Typically, in attempting to overcome this difficulty, as shown in the patent to E. G. Lundgren, U.S. Pat. No. 3,882,580, a special tapered chip-breaker is formed about the edge of the insert to provide a positive back rake angle to the detriment, however, of the stronger 90.degree. lip angle and at a considerable expense and effort required to generate the complicated chip-breaker so as not to deviate from the untouchable minimum "standard" relief angle of 6.degree..
Since the life of a plain bearing supported on a journal surface is directly related to the quality of its machined finish, the use of these inserts has been unsuitable up to the development of the present invention and conventional types of turning tools have been necessarily utilized to obtain high quality journal finishes.